Linear motion systems are in common use in industry, with different systems available to handle a variety of applications.
One type of system uses a ball and screw arrangement, in which the ball screw is rotated by a static rotary motor. An advantage of this arrangement is that the electric cables that drive the system are static, and therefore may be fixed to the main body of the machine. Some of the disadvantages however include limits in speed, and relatively high vibration, friction, and acoustic noise.
Where faster speed and smooth, high precision movements are required, electric motors having a stationary element and movable element are frequently employed. In some linear motors, for example, the movable element includes a current-carrying winding wrapped around a magnetic core of magnetizable material such as iron or steel, and the stationary element contains permanent magnets.
These linear motors have a disadvantage however in that the movable winding needs to be connected by cable to the driver current of the motor. In order to avoid deterioration of the connecting cables, a cable arrangement that is costly and complicated is usually required. Further, the cable connection creates mechanical friction and perturbations that affect the smoothness of the motor movement. An alternative type of linear electric motor reverses placement of the components, by placing the windings and magnetic core on the stator and the permanent magnets on the moving element. An example of this motor configuration is shown in US patent application US2007/0114854 to Miyamoto. A problem with this configuration, however, is that the windings and magnetic core are disposed all along the full length of the linear motor. Windings are usually wound around magnetic poles covering all the length of the electric motor. This makes the motor relatively heavy and expensive. Further, these motors have low efficiency since only the small section of the winding that is in front of the moving element is active.
Both of these common types of linear motor also have a strong attraction force between the moving and the static elements. The attraction force acts as a friction constraint on movement, requiring additional current input to overcome, which further reduces motor efficiency.
In my U.S. Pat. No. 9,252,650 (Villaret), there is described a linear motor that provides a transverse flux linear motor of high efficiency; in one embodiment, the moving carriage pushing force is provided by three magnetic circuits, each one having an opening inside which a row of magnets is inserted, and slide along the magnet row.
A feature of this configuration is to eliminate the need for moving cables.
Another feature is that the heating due to the thermal losses of the winding is not directly conducted to the carriage, resulting in a lower temperature.
However, the heat developed by the magnetic losses inside the magnetic material are still conducted to the carriage, resulting in a reduced but still problematic carriage temperature rise.
Another problem with this arrangement is that there are typically three (or at least two) rows of magnets. The pushing force is consecutively applied on the carriage at the respective opening of each row. This successive application of the pushing force to different positions of the carriage results in vibrations during the movement.
A further problem with this arrangement is the mechanical complexity. As will be shown, the extremities of the magnetic circuit are subject to strong and oscillating forces perpendicular to the movement path. This requires a stiff supporting frame to avoid vibrations. Implementing a stiff frame results in a heavy, complex and high cost structure.
Another problem with this arrangement is the mechanical mounting.
The assembly process is complex because the volume of space left between the rows is not accessible,
A further problem with this arrangement is that three magnet rows are required, thus adding cost.
Typical linear motors include current carrying coils wound around magnetic material. The magnetic material end sections called poles are moving on a linear path in proximity to a row of permanent magnets. The interaction between the magnetic field of magnets and the magnetic field in the pole proximity creates the working force. Typically, these motor poles are divided in three “phase” groups. A phase current of a three phase current generator is driven in each pole coil.
A limitation of this type of motor is that the number of poles per length unit is limited due to the size of the coils surrounding the poles. In order to produce a high force, it would be desirable to use a large number of poles, but coil size limits this number.
Therefore, these type of motors use large magnets and large poles, in order to be able to develop a sufficient working force. Large and strong permanent magnets have a high cost and are difficult to manipulate, which results in a high manufacturing cost.
Transverse flux motors make use of windings that extend along the movement path. Examples of that type linear motor are described in U.S. Pat. No. 5,854,521 by Nolle and U.S. Pat. No. 9,252,650 by Villaret. In these motors, the number of poles is not limited by the windings; this is because the same winding linear sections can extend over a large number of poles. It is thus possible to design the motor with a large number of poles, each pole being of small size. Consequently, permanent magnets are also of small size and lower cost. Furthermore, the same winding acts over all poles of the same phase, so that the number of windings is reduced to the number of phases. The winding shape is simpler and reduces the winding manufacturing cost.
A further advantage of the design presented in U.S. Pat. No. 9,252,650 by Villaret is that it is possible to make a linear motor without moving cables. The carriage does not need an electric feed. This improves the reliability and smoothness of movement. The cost of the moving cable arrangement is also avoided.
However, the linear motor described by Villaret still has the following disadvantages:
In a first aspect, the heat developed by the magnetic losses inside the magnetic material is still conducted to the carriage, resulting in some reduced but still problematic carriage temperature rise.
In a second aspect, there are typically three (or at least two) rows of magnets. The pushing force is consecutively applied on the carriage at the respective opening of each row. The successive applications of the pushing force to different lateral positions of the carriage create torsional torque and result in vibrations during the movement.
A further disadvantage of this linear motor is the mechanical complexity. As will be shown below, the extremities of the magnetic circuit are subject to strong and oscillating forces perpendicular to the movement path. This requires a stiff supporting frame to avoid vibrations. Implementation of a stiff frame results in a heavy, complex and high cost structure.
In another aspect of the mechanical mounting, the volume of space between the rows is not accessible, and this makes the assembly process complex. In order to make insertion of the winding possible, it is necessary to divide the magnetic circuits in several sections that can be re-assembled after inserting the winding. Assembling all these sections together, while the volume underneath the central part of the motor is not accessible, is a complex and thus costly procedure.
A further disadvantage of this embodiment is that three magnet rows are required, thus adding cost.